1. Field of the Invention
The present invention relates to an optical head device used for recording and/or reproducing information on and from an optical information storage medium such as an optical disc, respectively, and, more particularly, to a three-beam type optical device utilizing three light spots that are converged on the optical information storage medium to record or reproduce the information through one of the light spots and, at the same time, to detect the deviation of the recording/reproducing light spot from the center of an information track using the other two light spots.
2. Description of the Prior Art
An optical videodisc player is well known which operates to read a frequency modulated video and/or audio signal stored in the form of successively positioned depressed area, or "pits", on a plurality of concentric information tracks or a spiral information track carried by a video disc. The optical videodisc player employs an optical head system for directing a reading beam to impinge upon the information track and for gathering the reflected light modulated by the presence and absence of the pits on the information track. The optical videodisc player having an information recording capability in addition to the information reading capability is also well known.
An example of the prior art optical head system employed in the optical videodisc player is illustrated in FIG. 4(a), 4(b) and 4(c). Referring first to FIGS. 4(a) and 4(b) for the discussion of the prior art optical system, the system shown therein comprises a laser 1 employed in the form of a semiconductor laser, a plane-parallel optical element or beam splitter 40a having one of its opposite surfaces (the surface shown by 30) formed with a diffraction grating 30a adapted to diffract a light flux 2, emitted from the laser 1, to produce three diffracted light beams, and a condenser lens 5 for converging the diffracted light beams onto the information bearing surface of an optical disc 6 which has a series of successively positioned pits 7 formed in an information track 8 on that information bearing surface in the optical disc 6. The information track 8 on the information bearing surface of the optical disc 6 is in the form of, for example, a spiral groove of about 0.5 micrometers in width and 1.6 micrometer in pitch between the neighboring convolutions of the spiral groove. Information recorded on the optical disc 6 can be read out by directing a light spot 9 to impinge upon the information track, while the optical disc is rotated at a predetermined speed relative to the optical system and for gathering the reflected light modulated by the presence and absence of the pits on the information track.
The reflected light modulated by the presence and absence of the pits on the information track in the optical disc 6 can be detected, and converted into an electric signal, by a photo-detector assembly 10 after the reflected light has passed through the condenser lens 5 and then through the plano-parallel optical element 40a, and the electrical signal from the photo-detector assembly 10 is in turn supplied to subtractors 12 and 13 and an adder 16 shown in FIG. 4(b).
While the prior art optical head system is so constructed as hereinabove described, it operates in the following manner.
The light flux 2 emitted from the laser 1 is diffracted by the diffraction grating 30a, formed on the surface 30 of the plano-parallel optical element 40a, into the three light beams which are subsequently condensed by the condenser lens 5 so as to converge on the information bearing surface of the optical disc 6 thereby to form three light spots 9a, 9e and 9f as shown by respective hatched circles in FIG. 4(c). The optical head system is so designed and so positioned relative to the optical disc 6 that these light spots 9a, 9e and 9f can be lined up in a row inclined at a predetermined angle relative to the lengthwise direction of the invention track 8 as best shown in FIG. 4(c). In practice, the light spot 9a is used for actual information reading whereas the other light spots 9e and 9f are used for monitoring the position of the light spot 9a for the ultimate purpose of tracking error correction.
Rays of light reflected from the information bearing surface of the optical disc 6 are then passed back through the condenser lens 5 and also through the plano-parallel optical element 40a disposed at a predetermined angle of inclination required to direct the incoming light flux 2 towards the optical disc 6. The light rays having passed through the plano-parallel optical element 40a are imparted astigmatism such that separate line foci can be formed relative to the meridional light beam and the sagittal light beam as is well known to those skilled in the art, and are then projected onto the photo-detector assembly 10 as detected light spots 11a, 11e and 11f.
The photo-detector assembly 10 is so arranged and so positioning in such direction of optical axis that the zero-order diffracted light beam, that is, a primary one (corresponding to the light spot 9a) of the diffracted light beams reflected from the information bearing surface of the optical disc 6 and used for actual information reading, can form a circle of least confusion when the light spot 9a of the zero-order diffracted light beam is focused on the information track 8 on the optical disc 6. The photo-detector assembly 10 is of a construction wherein, as best shown in FIG. 4(b), the three reflected light beams can be detected at six different detecting areas. More specifically, the photo-detector assembly 10 comprises a four-segment detector divided into pairs of detector segments 10a and 10c, 10b and 10d adapted to monitor the zero-order light beam, and separate auxiliary detectors 10e and 10f positioned on respective sides of the four-segment detector and adapted to monitor the others of the diffracted light beams, that is, positive and negative first-order light beams.
As is well known to those skilled in the art, deviation of the light spot 9a from the information track 8 can be detected by calculating, with the use of the subtractor 13, the difference between respective outputs from the auxiliary detectors 10e and 10f. An output from the subtractor 13 is indicative of the occurrence of a tracking error and is, therefore, a tracking error signal which subsequently appears at an output terminal 14. This tracking error signal is in turn utilized to drive a tracking actuator (not shown) for brining the light spot 9a to the right position, where it is aligned with the information track 8, thereby to accomplish a tracking error correction.
The subtractor 12 is utilized to detect the difference between outputs from the paired detector segments 10a and 10c and the paired detector segments 10b and 10d to ascertain a focused condition of the light spot 9a on the information track 8. An output from the subtractor 12 which subsequently appears at an output terminal 15 is indicative of whether or not the light spot 9a is correctly focused on the information track 8 and, in the event that the light spot 9a is not correctly focused, that is, defocused, the output from the subtractor 12 is used to drive a focusing actuator (not shown) to bring the light spot 9a in a properly focused condition.
The detection of a deviation of the focal point is based on an astigmatic method. Specifically, when and so long as the light spot 9a projected onto the information bearing surface of the optical disc 6 is correctly focused, the detected light spot 11a on the detector segments 10a to 10d represents a circle of least confusion as shown in FIG. 4(b) and, therefore, a substantially circular shape. However, when a defocused condition occurs as a result of deviation in distance between the optical head system and the optical disc 6, the light spot 9a projected onto the information bearing surface of the optical disc 6 is deformed to render the detected light spot 11a to represent a generally elliptical shape. Accordingly, the actual defocused condition can be detected by electrically detecting the deformation of the light spot 11a.
The adder 16 operable to sum the outputs from the paired detector segments 10a and 10c, 10b and 10d of the four-segment detector together is utilized to reproduce the information recorded on the information track 8 in the optical disc 6, an output signal from such adder 16 being subsequently supplied to a well known signal processing circuit (not shown) for the eventual reproduction of the information.
The prior art optical head system has been found to have a problem associated with the information reproducibility. More specifically, an undesirable phenomenon which eventually adversely affects the information reproducibility tends to occur in view of the fact that the light beams, reflected from the information bearing surface of the optical disc 6 and traveling towards the photo-detector assembly 10, are diffracted during their passage through the plano-parallel optical element 40a. This problem resulting from the reflected light beams diffracted during the passage through the plano-parallel optical element 40a will now be discussed.
FIG. 5 illustrates a schematic diagram of the prior art optical head system shown in FIG. 4. In FIG. 5, reference characters e, m and f represents the negative first-order reflected and diffracted light beam, the zero-order reflected and diffracted light beam and the positive first-order reflected and diffracted light beam, respectively, all having been projected onto the information bearing surface of the optical disc 6.
The light beams reflected from the information bearing surface of the optical disc 6 are, after having passed through the plano-parallel optical element 40a, directed towards the photo-detector assembly 10 upon which they are incident as zero-order transmitted light beams E, M and F. However, in practice, during the passage through the plano-parallel optical element 40a, the reflected light beams are again diffracted to provide negative and positive first-order transmitted and diffracted light beams. In FIG. 5, the positive first-order transmitted and diffracted light beams are designed by E.sub.1, M.sub.1 and F.sub.1, and the negative first-order transmitted and diffracted light beams are designated by E.sub.-1, M.sub.-1 and F.sub.-1.
E.sub..+-.1 represents positive and negative first-order diffracted light beams resulting from the light spot e on the information bearing surface of the optical disc 6; M.sub..+-.1 represents positive and negative first-order diffracted light beams resulting from the light spot m; and F.sub..+-.1 represent positive and negative first-order diffracted light beams resulting from the light spot f. In FIG. 5, the light beams which ought to be incident upon the photo-detector assembly 10 should be the reflected light beams E, M and F, and these reflected light beams are projected onto the detector segments 10e, 10a to 10d and the detector 10f. However, of the positive and negative first-order transmitted and diffracted light beams resulting from the diffraction having taken place during the passage through the plano-parallel optical element 40a, the light beam M.sub.-1 overlaps with the light beam E, the light beams E.sub.1 and Fhd -1 overlap with the light beam M, and the light beam M.sub.1 overlaps with the light beam F, before they are received by the photo-detector assembly 10. Because these overlapped light beams are received by the photo-detector assembly 10 as hereinabove described, characteristics of the detection of the tracking error signal tend to be disturbed. This phenomenon will be discussed in detail with particular reference to FIG. 6 in which the optical disc 6 is shown as inclined at a minute angle .theta. relative to the position thereof shown in FIG. 5, have been pivoted about the point m at which the diffracted light beam is converged.
In the condition shown in FIG. 6, the light spot e is projected on the image bearing surface of the optical disc 6 at a position a distance .delta. e farther away from a reference position of the optical disc 6 shown in FIG. 5 (or as shown by the phantom line 51 in FIG. 6) whereas the light spot f is projected on the image bearing surface of the optical disc 6 at a position a distance .delta.f closer to the reference position of the optical disc 6 shown in FIG. 5. Accordingly, inclination of the optical disc 6 results in that the light spot E which is reflected from the optical disc 6 and subsequently incident upon the photo-detector assembly 10 has a change in phase corresponding to the optical length of 2 .delta. e, and the light spot F which is reflected from the optical disc 6 and subsequently incident upon the photo-detector assembly 10 has a change in phase corresponding to the optical length of -2.delta. f. On the other hand, the light spots M.sub.1 and M.sub.-1 do not have any change in phase because they are reflected from the spot m on the optical disc 6.
As a result thereof, a phase difference occurs between the light beams E and M.sub.-1 commonly incident upon the detector segment 10e and, therefore, interference occurs therebetween. A similar phase difference occurs between the light beams F and M.sub.1 commonly incident upon the detector segment 10f and, therefore, interference occurs therebetween. By these inteferences, the characteristics of detection of the tracking error tend to be disturbed. FIG. 7 illustrates a graph obtained during a simulated calculation to show how output signals from one of the detector segments 10e and 10f are disturbed by the above discussed interferences. During the simulated calculation which led to the results shown in the graph of FIG. 7, the following parameters were employed.
______________________________________ Ratio of Intensity of Light 0.2 Spots on Optical Disc (See FIG. 8) I.sub.+1r /I.sub.0r, I.sub.-1r /I.sub.0r Average Reflectivity of Optical Disc 0.8 Modulation Depth of Track Traversing Signal 0.23 Ratio of Intensity of Transmitted and 1.03 .times. 10.sup.-5 Diffracted Light Beams (See FIG. 8) I.sub.+1t /I.sub.0t, I.sub.-1t /I.sub.0t ______________________________________
In FIG. 7, a cycle of short variation corresponds to the traverse across the track, and a change in envelope shown by the broken lines corresponds to the inclination of the optical disc 6. As shown, assuming that the amplitude of the track traversing signal and the amplitude of variation of the envelope are expressed by A and B, respectively, the amount of variation of the envelope can be calculated by (B/A).times.100%. Considering that the tracking control accuracy required in the optical disc is about 0.1 micrometer and the track pitch, that is, the pitch between the neighboring tracks, is about 1.6 micrometer, it is desirable that the amount of change in track detection signal is not greater than about 10%. For this purpose, according to the result of the above discussed simulated calculation, it can be concluded that the ratio of intensity of the transmitted and diffracted light beams (positive and negative first-order/zero-order) must be not greater than 1.03.times.10.sup.-5.
However, according to the prior art optical head system, such a consideration has not been given attention to in designing and constructing the plano-parallel optical element 40a and, therefore, such a small ratio of intensity of the transmitted and diffracted light beams cannot be attained. This in turn brings about such a problem that the envelope of the tracking error signal tends to change beyond the tolerance.
For example, in the Japanese Laid-Open Patent Publication No. 61-151844 showing a typical prior art optical head system, no consideration is given to the problem associated with lowering the ratio of intensity of the transmitted and diffracted light beams.
In Japanese Laid-Open Patent Publication No. 55-101922 which pertains to a light beam divider for use in a photographic camera, not an optical head system, a technique to lower that ratio is disclosed. However, because the technique disclosed therein requires the formation over a substrate of a number of layers such as a reflection layer, and adhesive layer and an over-coated layer, the structure of the divider tends to be complicated.